1. Field of the Invention
The present invention pertains to the art of vehicles and, more particularly, to a hydraulic drive system for a vehicle.
2. Discussion of the Prior Art
Hydraulic drive systems for vehicles are known in the art. Most notably is the use of hydraulic drive systems on earthworking vehicles and other heavy machinery which operate in high torque/low speed environments. However, it has also been heretofore proposed to incorporate such drive systems on passenger vehicles.
In designing a new drive system for a passenger vehicle, one must carefully weigh the efficiency advantages of the drive system against its associated cost. Because of these considerations, initial attempts at such drive systems have generally concentrated on combining a conventional drivetrain with a hydraulic power system. More specifically, the internal combustion engine and transmission assembly is retained and the output thereof drives a pump which supplies the necessary fluid to the hydraulic power system. A major advantage of such an arrangement is that the vehicle can be readily assimilated into the marketplace since the driving characteristics, as seen from a driver of the vehicle, is essentially unchanged. Unfortunately, the cost associated with such vehicles is much higher than conventional internal combustion engine driven vehicles and therefore these hydraulically driven vehicles have not been commercially successful.
Other known system designs obviate the need for a conventional transmission and simply permit the internal combustion engine to directly drive the pump. In these known systems, a conventional accelerator or throttle member is controlled by the operator in order to adjust the operating speed of the engine and the engine is systematically started and stopped depending on sensed system pressure. Such known arrangements are disclosed, for example, in U.S. Pat. Nos. 4,098,083 and 4,350,220 issued to Carman. The problem with these known systems is that they generally require operator training and therefore cannot be readily assimilated into the marketplace, they are rather costly given the fact the internal combustion engines (or correspondingly sized electric motors) are large (generally commensurate in horsepower ratings to conventional vehicle engines) and they do not present a smoothly operating drivetrain that meet or exceed customary standards. For instance, the periodic starting and stopping of the engines as disclosed in the '083 and '220 patents create undesirable disturbances.